This invention relates generally to woven fabrics, and relates more specifically to woven fabrics for papermakers.
In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper xe2x80x9cstockxe2x80x9d) is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rollers. The belt, often referred to as a xe2x80x9cforming fabricxe2x80x9d, provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity alone or with assistance from one or more suction boxes located on the lower surface (i.e., the xe2x80x9cmachine sidexe2x80x9d) of the upper run of the fabric.
After leaving the forming section, the paper web is transferred to a press section of the paper machine, in which it is passed through the nips of one or more pairs of pressure rollers covered with another fabric, typically referred to as a xe2x80x9cpress felt.xe2x80x9d Pressure from the rollers removes additional moisture from the web; the moisture removal is often enhanced by the presence of a xe2x80x9cbattxe2x80x9d layer on the press felt. The paper is then conveyed to a drier section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.
Typically, papermaker""s fabrics are manufactured as endless belts by one of two basic weaving techniques. In the first of these techniques, fabrics are flat woven by a flat weaving process, with their ends being joined to form an endless belt by any one of a number of well-known joining methods, such as dismantling and reweaving the ends together (commonly known as splicing), or sewing a pin-seamable flap on each end or a special foldback, then reweaving these into pin-seamable loops. In a flat woven papermaker""s fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction. In the second technique, fabrics are woven directly in the form of a continuous belt with an endless weaving process. In the endless weaving process, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. As used herein, the terms xe2x80x9cmachine directionxe2x80x9d (MD) and xe2x80x9ccross machine directionxe2x80x9d (CMD) refer, respectively, to a direction aligned with the direction of travel of the papermaker""s fabric on the papermaking machine, and a direction parallel to the fabric surface and traverse to the direction of travel. Both weaving methods described hereinabove are well known in the art, and the term xe2x80x9cendless beltxe2x80x9d as used herein refers to belts made by either method.
When tissue paper is being formed, typically a forming fabric with a top surface having a relatively fine mesh is employed. Such a fabric can reduce the tendency of the paper to have xe2x80x9cpin holes.xe2x80x9d Also, because tissue paper is typically formed on paper machines that run at high speeds (as much as 6,000 feet per minute) with high permeability fabrics, tissue forming fabrics generally are relatively thin (i.e., of low caliper); low caliper fabrics tend to drain quickly (due to a lesser void volume) and are therefore often more effective than thicker fabrics. In addition, the high operating rate of a tissue forming machine can also cause the xe2x80x9csheet releasexe2x80x9d properties of a tissue forming fabric to be very important. In contrast to forming fabrics for higher grades of paper, such as printer paper, the xe2x80x9ccoplanarityxe2x80x9d of the papermaking surface is generally less critical for tissue forming fabrics, as a fabric having less coplanar surface can produce a softer paper (which is, of course, an important quality in tissue). Further, a typical tissue forming fabric is xe2x80x9cCMD-predominantxe2x80x9d on its top surface; i.e., the CMD yarns of the top surface tend to comprise more of the top surface than do the MD yarns and provide much of the fiber support for the paper stock fibers.
The foregoing demonstrates that forming fabrics for tissue paper can have much different performance characteristics than those for forming newsprint, printer paper, and other finer paper grades. As such, forming fabrics suitable for fine paper grades, such as those illustrated in U.S. Pat. Nos. 4,987,929; 5,518,042; and 5,937,914 to Wilson, may not be suitable for the formation of tissue paper. Fabrics that are designed for tissue paper formation, such as those illustrated in U.S. Pat. Nos. 5,025,839 to Wright and 5,857,498 to Barreto, can have insufficient permeability under certain circumstances. One other proposed tissue forming fabric, that illustrated in U.S. Pat. No. 5,421,374 to Wright, addresses the permeability issue by including only one bottom CMD yarn for every three top CMD yarns, but this fabric can have problems with the top CMD yarns bunching together such that there is not uniform spacing between the top CMD yarns.
In view of the foregoing, it is an object of the present invention to provide a papermaker""s fabric suitable for forming tissue paper.
It is another object of the present invention to provide a tissue forming fabric that addresses the permeability and top CMD spacing uniformity problems described above.
These and other objects are satisfied by the present invention, which includes a papermaker""s fabric that can provide suitable permeability, surface structure, and top CMD yarn spacing for tissue formation. The papermaker""s fabric comprises: a base fabric structure including machine direction yarns and primary cross machine direction yarns interlacing to form a papermaking surface, wherein the machine direction yarns and primary cross machine direction yarns are interlaced in a pattern having a harness repeat of greater than two. The fabric also includes first and second companion cross machine direction yarns positioned between each pair of adjacent primary cross machine direction yarns. Each first companion cross machine direction yarn has an interlacing pattern relative to the machine direction yarns that is identical to a first of the pair of adjacent primary cross machine direction yarns, and each second companion cross machine direction yarn has an interlacing pattern relative to the machine direction yarns that is identical to a second of the pair of adjacent primary cross machine direction yarns. The first companion cross machine direction yarn is positioned between the second primary and companion cross machine direction yarns. The primary cross machine direction yarns have a first diameter, and the first and second companion yarns have a second diameter, wherein the ratio between the first and second diameters is between about 0.9 and 1.1. In this configuration, the companion paper side cross machine direction yarns tend to remain separated from each other, thereby providing a surface structure and a permeability that can suitably produce tissue paper.
In another embodiment, a papermaker""s fabric of the present invention comprises machine direction yarns interwoven with first and second sets of top cross machine direction yarns and a bottom set of cross machine direction yarns in a repeat unit. The first and second sets of top cross machine direction yarns are arranged in alternating fashion such that one of the first set of cross machine direction yarns resides between two of the second set of top cross machine direction yarns, and one of the second set of top cross machine direction yarns resides between two of the first set of top cross machine direction yarns. Each of the machine direction yarns of the repeat unit passes below at least one of the bottom machine direction yarns, and each of the machine direction yarns passes either (a) above two adjacent top cross machine direction yarns of the first set and below one of the top cross machine direction yarns of the second set positioned therebetween, or (b) above two adjacent top cross machine direction yarns of the second set and below one of the top cross machine direction yarns of the first set positioned therebetween. This configuration can also provide a surface structure and permeability suitable for forming tissue paper. It is preferred that, in this configuration, the paper side cross machine direction yarns comprising the first set be formed of a material that has a different elastic modulus (i.e., differs in flexibility) than the material from which the paper side cross machine direction yarns are formed.